It is generally known to produce, for model constructional sets, constructional elements, of any desired shape, which comprise a substantially hollow plastic member in the form of a box which has no base and which is furnished, on its upwardly turned side, with connecting members for clamping on to the lower part, of the inwardly turned side surfaces, of other elements. By such co-operation one element is clamped on to another with adequate force, so that a number of constructional units thus assembled form a coherent structure, which can then be dismantled again by removing the individual elements.
Austrian Patent Specification No. 184 497 discloses a plastic constructional element, for decorative purposes and toys, comprising a member elongate in plan in the form of a box which is open at the base and which has disposed in a row in the longitudinal direction on its top side connecting members in the form of pegs tapering slightly conically upwards, and whose interior has two inwardly directed, perpendicularly extending clamping elements which are disposed on the inner surface of the side wall and can be moved into engagement with depressions in the pegs. The pegs formed with depressions are constructed in star shape, enabling the constructional units to be interconnected so that their longitudinal axes enclose angles different from 90.degree..
One disadvantage of this type of constructional element derives from the fact that when the angular position of the constructional units in relation to one another is altered, the upper unit must be removed from the peg of the subjacent unit and then replaced in the required position. The result is that there are no possible variations in turning a constructional unit once applied. Furthermore, this prior art constructional unit has no inner transverse walls. It is clamped via slightly conical ribs, disposed on the inside of the side wall and the also slightly conically extending flanks of the stellate jagged points of the peg. If the walls are too thin, or clamping takes place not only in the clamping zone of the flanks, but in the depressions of the stellate projections, the result is that the side walls of the unit bulge unattractively.
German Auslegeschrift No. 11 06 222 also discloses a non-rotatable plastic constructional element, comprising a member elongate in plan in the form of a box which is open at the bottom and whose top side has connecting members taking the form of cylindrical pegs extending in the longitudinal direction, its interior having inner walls extending from the cover member transversely of the longitudinal direction of the constructional element and are connected in one piece only to the cover member, but not to the side walls. The clamping principle of those constructional units is that the connecting members or pegs enter into clamping engagement with a resilient transverse wall, which is connected to the cover member, but not to the side walls of the box-shaped constructional unit and the particular end wall. A disadvantage of these toy constructional units is that they are very difficult to separate from one another, especially if the units are fairly large, and moreover they can be assembled only in alignment or transversely. The side walls do not bulge, however, since the resilient transverse wall is not connected to the side wall.